1. Field of the Disclosure
This document relates to a display device having touch sensors embedded in a display panel.
2. Related Art
A user interface (UI) enables interactions between humans (users) and various kinds of electric and electronic equipment so that the users can control the equipment easily as they want. Typical examples of the user interface include keypads, keyboards, mouses, on-screen displays (OSD), and remote controllers having an infrared communication function or a radio frequency (RF) communication function. The user interface technology has continuously expanded to enhance user's sensibility and ease of operation. The user interface has been recently developed to include touch UI, voice recognition UI, 3D UI, etc.
The touch UI has been indispensably used in portable information appliances and has been expanded to the use of home appliances. Capacitive touch sensing systems give higher durability and sharpness compared to the existing resistive ones, and can be applied to a wide variety of applications.
Touch sensors in the touchscreen may be placed on a display panel or embedded in the display panel. A display driving circuit comprises a data driver that generates a data voltage and a gate driver that generates a gate pulse (or scan pulse) in synchronization with the data voltage. With the touch sensors embedded in the display panel, the touch sensors and the pixels are electrically coupled, and a signal applied to the pixels may therefore act as noise on the touch sensors. In the case where the touch sensors are embedded in the display panel, 1 frame period for the display panel may be time-divided into a pixel driving period for writing input image data to the pixels and a touch sensing period for driving the touch sensors, in order to reduce the mutual influence of the pixels and the touch sensors on each other.
The 1 frame period for the display panel may be time-divided into a plurality of pixel driving periods and a plurality of touch sensing periods. When the screen of the display panel is divided into first and second blocks B1 and B2, as shown in FIG. 1, input image data is written to the pixels of the first block B1 during a first pixel driving period Td1 and then the touch sensors across the entire screen are driven to sense touch input during a first touch sensing period Tt1, as shown in FIG. 2. Subsequently, input image data is written to the pixels of the second block B2 during a second pixel driving period Td2 and then the touch sensors across the entire screen are driven to sense touch input during a second touch sensing period Tt2.
The gate driver sequentially shifts a gate pulse applied to gate lines by using a shift register. The gate pulse is synchronized with an input image data voltage, i.e., pixel voltage, and sequentially selects pixels to be charged with the data voltage line by line. The shift register comprises stages connected in cascade connection. Each of the stages operates on a D flip-flop basis. Accordingly, the stages produce output upon receiving a start signal or an output from a previous stage as the start signal and receiving a clock.
If there is a touch sensing period Tt1 between the first pixel driving period Td1 and the second pixel driving period Td2, as shown in FIG. 2, the first gate pulse voltage for the second pixel driving period Td2 gets lower and the brightness of the pixels on the first pixel line of the second block B2 becomes different from the brightness of those on the other pixel lines. Due to this, a dim line appears on the boundary between the blocks B1 and B2. This is because the Q node voltage of the shift register that outputs a first gate pulse for the second block B2 is inverted during the touch sensing period Tt1, thus lowering the output voltage of the stages. The touch sensing period Tt1 is much longer than 1 horizontal period. Since the decay time becomes longer as the touch sensing period Tt1 becomes longer, the amount of decay in the Q node voltage of the stages becomes larger. Accordingly, the embedding of touch sensors into a display panel and the time-division of 1 frame period for the display panel into a plurality of pixel driving periods and a plurality of touch sensing periods may cause degradation of the output characteristics of the gate driver. In the conventional art, a touch sensing circuit outputs coordinate data of a single touch input during 1 frame period. Consequently, a general display device's touch report rate is equal to the frame rate for input images.